The present invention relates to a color picture tube and, more specifically to a shape of a panel provided with a phosphor screen.
In a conventional color picture tube, a glass panel with a phosphor screen was commonly spherical for purpose of making the color picture tube lightweight and the like. However, by reason of development of a simulation technology in recent years and the like, it has become possible to produce a panel of a substantially flat shape.
In an actual color picture tube having a flat plate panel with parallel surfaces, however, it is recognized that portions of an image around edges of a screen appear to be floating in a greater degree toward a viewer than a portion of the image around a center of the screen, so that an image of a human face displayed on the screen, for example, which naturally should be generally elliptical, is prone to appear to be somewhat crescent-shaped.
It may be considered that the above-mentioned phenomenon occurs for the following reason. As shown in FIG. 9 schematically showing a vertical sectional view of the conventional panel, a viewer 30 views around a center of a panel 31 at a substantially right angle with respect to an outer surface of the panel 31. On the other hand, the viewer 30 views around an edge of the panel 31 obliquely at an angle .alpha. with respect to a Z-axis. Further, if a thickness of the panel 31 around its center when viewed in a direction normal to the center of an outer surface 31b of the panel 31 is represented by T.sub.0, it can be seen that a thickness T.sub.1 of the panel 31 around its edge when viewed in an oblique direction at the angle .alpha. with respect to the Z-axis, is larger than T.sub.0. Accordingly, as a point being viewed by the viewer 30 approaches the edge of the screen, apparent floating distortion of the image displayed in an image display area on an inner surface 31a of the panel 31 (or on a phosphor screen 32) becomes greater.
As a more specific example, let us assume that a refractive index n of glass constituting the panel 31 is 1.536, size of the image display area on the panel 31 in a diagonal direction is 260 [mm], and the viewer 30 views the phosphor screen from a position 95 [mm] away from the outer surface 31b of the panel 31. Then, the phosphor screen recognized by the viewer 30, which is hereinafter referred to as an apparent screen 33 because it is seen as if it were located closer to the viewer 30 than the actual phosphor screen 32, is positioned at a depth of T.sub.0 /n (.apprxeq.2T.sub.0 /3) from the outer surface 31b at the center of the panel 31, that is to say, at a position of approximately T.sub.0 /3 floating from the inner surface 31a at the center of the panel 31. As a point being viewed by the viewer 30 approaches an edge of the screen, the apparent floating distortion of the apparent screen 33, which is expressed by (T.sub.0 /3+.DELTA.TD), becomes larger.
FIG. 10 shows graphs indicating results of calculations of a relationship between the viewing angle .alpha. degrees with respect to the Z-axis corresponding to a position of a point in the image display area of the panel 31 and an increment .DELTA.TD of the apparent floating distortion of the image. Referring to FIG. 10, a radius of curvature of an outer surface of the panel is designated by RP [mm] and is calculated on the assumption that an inner surface of the panel is flat. However, similar results can also be obtained if the outer surface of the panel is flat and the inner surface of the panel is curved. It is also assumed in FIG. 10 that the viewer's eyes are positioned 95 [mm] away from the outer surface of the panel. The case in which RP=90000 [mm] corresponds to the case in which the panel is a flat plate with parallel surfaces. In this case, it can be seen from FIG. 10 that a portion of an image located at the viewing angle .alpha. of 50 degrees, for example, appears to be floating .DELTA.TD, which is approximately 2.4 [mm], toward the viewer, compared to a portion of the image around the center of the phosphor screen.